Most women have a 28-day cycle and ovulate on day 14. However, the odds change across age groups — you most likely have:
- a 29-day cycle with ovulation on day 15 between 18 and 24 years
- a 28-day cycle with ovulation on day 14 between 25 and 39 years
- a 27-day cycle with ovulation on day 12 above 40 years
These are the findings of a 2020 study that analyzed 18,761 cycles using a mobile app and LH urine tests.1
More importantly, the most fertile day is 2 days before ovulation. For centuries, we thought that ovulation occurred on the most fertile day. In reality, you are about five times more likely to get pregnant two days before ovulation than on the day of ovulation itself — a big difference.
Ovulation
At ovulation, the egg matures, the follicle ruptures, and the ovary releases the egg. It then travels toward the uterus through the fallopian tube. If sperm cells are waiting, one of them can fertilize the egg. Otherwise the egg dissolves within 12–24 hours. If it is fertilized, an embryo begins to develop and can implant in the uterus about a week later.
Actual day of ovulation
A large 2020 study of 32,595 women and 75,981 cycles used luteinizing hormone (LH) urine tests to pinpoint ovulation.2 When LH rises in the urine, ovulation follows the next day. Most women had a 28-day cycle and ovulation on day 15, but the researchers found that the actual day of ovulation depends heavily on cycle length and varies considerably within it. The calendar method alone cannot give an accurate forecast.
Drag the slider below to see how the most likely ovulation day shifts with cycle length — and how much spread there is even within a single cycle length.
Signs of ovulation
In humans, the fertile period is hidden. There are no obvious signs, and it is hard to say exactly when ovulation happens. Still, sexual desire, skin, breast sensitivity, breathing, heartbeat, smell, movement and appearance may all shift as it approaches. During follicular rupture some women feel a stinging sensation near the ovary — mid-cycle pain — sometimes with dizziness, headache or light spotting.3
The signs and tests fall into two groups: those that appear before ovulation and warn that the fertile window is opening, and those that appear after ovulation and confirm that it has happened. For getting pregnant, only the first group is useful in real time — by the time the second group signals, the fertile window is already closing.
Signs before ovulation — predicting the fertile window
Cervix
The position and softness of the cervix change as ovulation approaches: it becomes higher, softer and more open. The cervix produces cervical mucus, which together with the mucus itself is the most reliable sign of fertile days. Conception needs not only sperm and egg but also fertile mucus so that sperm can swim through the cervix and survive in the fallopian tube.
Cervical mucus
Under the influence of estrogen the cervix produces stretchy, slightly alkaline, nourishing mucus — often compared to raw egg white — in which healthy sperm can swim up and survive for 3 days (rarely up to 5). Its appearance a few days before ovulation is the earliest reliable in-body sign of the fertile window.3
Saliva
In most women, fern-shaped crystals appear in a dried saliva sample during the fertile phase and disappear once the egg matures. Because the crystals show up several days before ovulation — during the highest-fertility days — the saliva test flags the fertile window early enough to act on it. It does not work for everyone: anything that affects estrogen levels (age, medications, infections, breastfeeding, pregnancy) can change the pattern.
Babyndex automates this test — you take the sample with a small saliva microscope, and the app recognises the fern pattern for you and logs it across your cycle.
LH urine test
Urine test strips detect the surge of luteinizing hormone (LH) that triggers ovulation. The LH surge starts in the blood at dawn and shows up in urine about half a day later, with ovulation following 24–36 hours after that.3 LH tests are widely available and accurate at catching the surge, but they warn you very late — usually on the day before ovulation, when only one fertile day is left.
Ultrasound
Transvaginal ultrasound lets a clinician see the ovaries, measure the growing follicles, identify the dominant one, and watch the egg being released. It is the most accurate way to time ovulation, but it needs a specialist and repeated visits, so it is used mainly in fertility clinics rather than for everyday tracking.
Signs after ovulation — confirming it happened
These signs are useful for confirming that a cycle was ovulatory, checking the length of the luteal phase, or ruling out anovulation — but they appear too late to time intercourse for the current cycle.
Basal body temperature (BBT)
After ovulation, progesterone from the corpus luteum raises the basal body temperature by a few tenths of a degree. Measured first thing in the morning with a sensitive thermometer, a sustained rise across 2–3 days confirms ovulation has occurred. BBT is cheap and simple, but it only tells you about ovulation in hindsight and works best for people with regular cycles.3
Progesterone (PdG) urine test
After ovulation, progesterone rises and its metabolite pregnanediol glucuronide (PdG) is excreted in urine.4 Home tests such as Proov measure PdG across the luteal phase to confirm that ovulation actually happened and that progesterone stayed high long enough to support a possible pregnancy. Like BBT, this is a post-ovulation confirmation — helpful for diagnosing anovulatory or short-luteal cycles, not for planning intercourse in real time.
References
- Grieger JA, Norman RJ (2020). Menstrual Cycle Length and Patterns in a Global Cohort of Women Using a Mobile Phone App: Retrospective Cohort Study. J Med Internet Res, 22(6):e17109 — 18,761 cycles tracked with a mobile app and LH urine tests.
- Bull JR et al. (2019). Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. npj Digital Medicine, 2:83 — large-scale analysis of cycle and ovulation timing using LH urine tests.
- Su HW, Yi YC, Wei TY, Chang TC, Cheng CM (2017). Detection of ovulation, a review of currently available methods. Bioengineering & Translational Medicine, 2(3):238–246 — comparative review of BBT, LH, cervical mucus, saliva ferning, ultrasound and progesterone-based ovulation detection.
- Ecochard R et al. (2013). Use of urinary pregnanediol 3-glucuronide to confirm ovulation. Steroids, 78(10):1035–1040 — PdG in urine as a reliable post-ovulation confirmation marker (basis of at-home tests such as Proov).